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Lula's wide-ranging, 45-minute speech to cheering supporters focused broadly on defeating Bolsonaro and improving the economic conditions of the working class, although he diverged frequently to talk about everything from his fiancé to solidarity with leftist governments in Bolivia and Venezuela.
Lula, who was president from 2003 to 2010, also took aim at a long list of political enemies, including Bolsonaro, Economy Minister Paulo Guedes and Justice Minister Sergio Moro, a former judge who initially ruled to convict Lula.
"I want to tell them, I'm back," the 74-year-old told hundreds of supporters dressed in red, the color of his Workers Party, outside of the metalworkers union where he got his political start.
He said Guedes seeks to remake Brazil economically in the image of Chile, long seen as a model of financially conservative governance, but that those policies are the reason for the widespread street protests paralyzing its Latin American neighbor.
A judge ordered that Lula be freed on Friday, a day after Brazil's Supreme Court issued a broader ruling ending the mandatory imprisonment of convicted criminals after they lose their first appeal. Lula had been imprisoned on a corruption conviction carrying a nearly nine-year sentence.
Earlier on Twitter, the president called for supporters to rally around his government's agenda, which has included a severe tightening of public spending, saying that they must not allow Brazil's next phase of recovery to be derailed.
"Do not give ammunition to the scoundrel, who is momentarily free but full of guilt," Bolsonaro said.
While Bolsonaro did not mention Lula by name, his left-wing rival took direct aim at the president.
"If we work hard, in 2022 the so-called left that Bolsonaro is so afraid of will defeat the ultra-right," he said.
Lula, who left the presidency with sky-high approval ratings, is ineligible to stand for office until 2025 under Brazil's "Clean Record" law because of a conviction for taking bribes. But his release is expected to energize the left ahead of next year's municipal elections.
He was imprisoned in 2018 after being found guilty of receiving bribes from construction companies in return for public contracts.
Lula has maintained his innocence. On Saturday he repeated that Justice Minister Moro, prosecutors and police were lying about his guilt for political reasons.
"(I'm) not responding to criminals, jailed or freed. Some people deserve to be ignored," Moro responded on Twitter.